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Magnetic resonance method and apparatus for acquisition of image data of a vessel wall

Magnetic resonance method and apparatus for acquisition of image data of a vessel wall description/claims

1. Field of the Invention

The present invention concerns a method for acquisition of image data of a vessel wall by means of magnetic resonance technology as is used for diagnosis of variations of the vessel wall that are due to an atherosclerosis. The invention also concerns a magnetic resonance apparatus for implementing such a method.

2. Description of the Prior Art

Arteriosclerosis is a systemic illness of the arteries that leads to deposits of blood lipids, thromboses, connective tissue and calcium in the vessel walls. The focal variations that occur in the inner and in the middle vessel wall are also referred to as atherosclerosis. The atherosclerotic variations are often locally limited and form what are known as plaques. Among other things, heart infarcts and strokes are among the typical results of arteriosclerosis.

Thromboembolic events, i.e. the formation of a blood clot in an artery, are often based on a rupture of a “vulnerable” plaque, namely the tearing of the thin fibrous cap of the inflamed vessel wall variation.

The vulnerability of a plaque appears to be substantially more significantly influenced by the tissue composition of the plaque than by the size of the plaque and the remaining size of the vessel lumen. Primarily calcium deposits (calcified tissue), connective tissue, lipid deposits and fibrin deposits are among the tissue components of a plaque.

A number of methods exist in order to be able to examine a vessel wall variation.

Intravascular ultrasound (IVUS) allows a radiation-free examination of the vessel wall, and is predominantly suitable for soft, non-calcified plaque, but is an invasive examination method and is relatively expensive.

Examination methods based on computed tomography entail a relatively high radiation exposure for the patient to be examined.

Magnetic resonance (MR) technology is also used for diagnosis of arteriosclerosis. The MR technique is a technique known for some decades with which images of the inside of an examination subject can be generated. Greatly simplified, to generate an MR image the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla and more) in an MR apparatus so the nuclear spins thereof orient along the basic magnetic field. Radio-frequency excitation pulses are radiated into the examination subject to excite nuclear magnetic resonances, the triggered nuclear magnetic resonances being measured (deleted) and MR images being reconstructed therefrom. For spatial coding of the measurement data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored in a mathematical organization called a k-space matrix as complex numerical values. By multi-dimensional Fourier transformation, an MR image can be reconstructed from the data in the k-space matrix. The temporal series of the excitation pulses and the gradient fields for excitation of the image volume to be measured, for signal generation and for spatial coding is designated as a sequence (or also as a pulse sequence or measurement sequence).

The MR technique is also used for angiography by the application of specific sequences. MR angiography is used for examination of the lumen of a vessel and thus for detection of a possibly present stenosis. The size of the lumen, however, does not correlate with the vulnerability of a plaque to rupture, which is why at-risk patients can be only insufficiently identified with this examination method.

One possibility to be able to quantify atherosclerotic vessel variations is described in the document by J. M. A. Hofman et al., “Quantification of Atherosclerotic Plaque Components Using In Vivo MRI and Supervised Classifiers”, Magn. Res. Med. 55(4), 790-799, 2006. Various T1-weighted, T2-weighted and proton density-weighted sequences are used for image acquisition of atherosclerotic vessel wall variations. Further analysis of this approach has shown that calcifications and/or calcium deposits in a plaque can be only insufficiently detected since calcium, due to its short T2 relaxation time, appears in the image as a region with signal attenuation. Signal attenuations, however, can also be based on various artifacts, such that calcifications are often overestimated.

A sequence known as an ultrashort echo time sequence (UTE sequence in the following) with which signals of tissue components with a short T2 relaxation time can be measured before the transverse magnetization has decayed, is disclosed in WP 2005/026748 and in the article by P. D. Gatehouse and G. M. Bydder, “Magnetic Resonance Imaging of Short T2 components in Tissue”, Clin Radiol 58(1), 1-19, 2003.

An object of the present invention is to provide a method for acquisition of an image of a vessel wall that enables a non-invasive, x-ray-free and high-resolution image acquisition with which an image of an atherosclerotic vessel wall variation can be acquired. The method should allow an improved evaluation of the composition of the vessel wall variation and an improved identification of patients at risk for a thromboembolic event. Furthermore, it is an object of the invention to provide a magnetic resonance apparatus for implementation of such a method.

The above object is inventively achieved by a method for acquisition of an image for examination of a vessel wall variation according to the invention, including positioning a vessel wall section of a patient to be examined in an imaging volume of a magnetic resonance apparatus, acquisition of image data of the vessel wall section with an ultrashort echo time sequence, and generating an image from the acquired image data.

A suitable ultrashort echo time sequence is described in WO 2005/026748 and in the document by P. D. Gatehouse and G. M. Bydder, “Magnetic resonance Imaging of Short T2 Components in Tissue”, Clin Radiol 58(1), 1-19, 2003. A UTE sequence is characterized by an echo time TE of less than 100 μs (microseconds), advantageously less than 80 μs.

The imaging with an ultrashort echo time sequence is based on a short, advantageously non-selective RF excitation pulse with subsequent acquisition of signals from excited nuclear spins. In order to enable the desired short echo times, the acquisition of the measurement data already ensues during the ramp phase while the gradient fields switched for acquisition of the measurement data are being established.

In a three-dimensional ultrashort echo time sequence, for example, gradient fields are radiated that enable an asymmetrical acquisition of the measurement data from the center of k-space radially outwardly—for example to a surface of a sphere in k-space.

It is possible to also measure signals of tissue components with a short T2 relaxation time (such as, for example, calcified tissue) so that this tissue also generates a positive contrast (i.e. a visible signal) in the image. In the generated image this is advantageous since now calcifications (which generate only a negative contrast with conventional MR sequences, i.e. generate only an insufficient signal in the representation) can be made visible. The generated image allows a user to better assess the composition of a vessel wall variation. Computer-aided evaluation algorithms based on the generated image data can likewise implement a more precise quantification of tissue components of a plaque since now one of the components that is essential for a diagnosis of the vulnerability of a plaque, namely calcifications or calcium deposits, generates a distinctly visible and measurable signal.

In an embodiment, the ultrashort echo time sequence includes at least one radio-frequency saturation pulse for suppression of signals of nuclear spins of fat tissue. In this embodiment, it is possible to reduce signals that have their origin in nuclear spins of fat tissue since these nuclear spins are saturated by the radio-frequency saturation pulse. The contrast between lipid deposits and calcifications in a vessel wall hereby increases.

In another embodiment, the ultrashort echo time sequence includes at least one radio-frequency saturation pulse for suppression of signals of nuclear spins whose T2 relaxation time is greater than a predetermined threshold. It is thereby possible to reduce signals that have their origin in tissue with a long T2 relaxation time. In the generated image this causes a higher contrast between this type of tissue and calcified tissue.

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